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Register a new userEvidence for dark matter in the inner Milky Way...Really?
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The following is a comment on the recent letter by Iocco et al. (2015, arXiv:1502.03821) where the authors claim to have found ...convincing proof of the existence of dark matter.... The letter in question presents a compilation of recent rotation curve observations for the Milky Way, together with Newtonian rotation curve estimates based on recent baryonic matter distribution measurements. A mismatch between the former and the latter is then presented as evidence for dark matter. Here we show that the reported discrepancy is the well known gravitational anomaly which consistently appears when dynamical accelerations approach the critical Milgrom acceleration a_0 = 1.2 times 10^{-10} m / s^2. Further, using a simple modified gravity force law, the baryonic models presented in Iocco et al. (2015), yield dynamics consistent with the observed rotation values.
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This is a brief rebuttal to arXiv:1502.03821, which claims to provide the first observational proof of dark matter interior to the solar circle. We point out that this result is not new, and can be traced back at least a quarter century.
We use hydrodynamical simulations to construct a new coherent picture for the gas flow in the Central Molecular Zone (CMZ), the region of our Galaxy within $Rleq 500, mathrm{pc}$. We relate connected structures observed in $(l,b,v)$ data cubes of molecular tracers to nuclear spiral arms. These arise naturally in hydrodynamical simulations of barred galaxies, and are similar to those that can be seen in external galaxies such as NGC4303 or NGC1097. We discuss a face-on view of the CMZ including the position of several prominent molecular clouds, such as Sgr B2, the $20,{rm km, s^{-1}}$ and $50,{rm km, s^{-1}}$ clouds, the polar arc, Bania Clump 2 and Sgr C. Our model is also consistent with the larger scale gas flow, up to $Rsimeq 3,rm kpc$, thus providing a consistent picture of the entire Galactic bar region.
The unambiguous detection of Galactic dark matter annihilation would unravel one of the most outstanding puzzles in particle physics and cosmology. Recent observations have motivated models in which the annihilation rate is boosted by the Sommerfeld effect, a non-perturbative enhancement arising from a long range attractive force. Here we apply the Sommerfeld correction to Via Lactea II, a high resolution N-body simulation of a Milky-Way-size galaxy, to investigate the phase-space structure of the Galactic halo. We show that the annihilation luminosity from kinematically cold substructure can be enhanced by orders of magnitude relative to previous calculations, leading to the prediction of gamma-ray fluxes from up to hundreds of dark clumps that should be detectable by the Fermi satellite.
We analyse systems analogous to the Milky Way (MW) in the EAGLE cosmological hydrodynamics simulation in order to deduce the likely structure of the MWs dark matter halo. We identify MW-mass haloes in the simulation whose satellite galaxies have similar kinematics and spatial distribution to those of the bright satellites of the MW, specifically systems in which the majority of the satellites (8 out of 11) have nearly co-planar orbits that are also perpendicular to the central stellar disc. We find that the normal to the common orbital plane of the co-planar satellites is well aligned with the minor axis of the host dark matter halo, with a median misalignment angle of only $17.3^circ$. Based on this result, we infer that the minor axis of the Galactic dark matter halo points towards $(l,b)=(182^circ,-2^circ)$, with an angular uncertainty at the 68 and 95 percentile confidence levels of 22$^circ$ and 43$^circ$ respectively. Thus, the inferred minor axis of the MW halo lies in the plane of the stellar disc. The halo, however, is not homologous and its flattening and orientation vary with radius. The inner parts of the halo are rounder than the outer parts and well-aligned with the stellar disc (that is the minor axis of the halo is perpendicular to the disc). Further out, the halo twists and the minor axis changes direction by $90^circ$. This twist occurs over a very narrow radial range and reflects variations in the filamentary network along which mass was accreted into the MW.
Stellar streams produced from dwarf galaxies provide direct evidence of the hierarchical formation of the Milky Way. Here, we present the first comprehensive study of the LMS-1 stellar stream, that we detect by searching for wide streams in the Gaia EDR3 dataset using the STREAMFINDER algorithm. This stream was recently discovered by Yuan et al. (2020). We detect LMS-1 as a $60deg$ long stream to the north of the Galactic bulge, at a distance of $sim 20$ kpc from the Sun, together with additional components that suggest that the overall stream is completely wrapped around the inner Galaxy. Using spectroscopic measurements from LAMOST, SDSS and APOGEE, we infer that the stream is very metal poor (${rm langle [Fe/H]rangle =-2.1}$) with a significant metallicity dispersion ($sigma_{rm [Fe/H]}=0.4$), and it possesses a large radial velocity dispersion (${rm sigma_v=20 pm 4,km,s^{-1}}$). These estimates together imply that LMS-1 is a dwarf galaxy stream. The orbit of LMS-1 is close to polar, with an inclination of $75deg$ to the Galactic plane. Both the orbit and metallicity of LMS-1 are remarkably similar to the globular clusters NGC 5053, NGC 5024 and the stellar stream Indus. These findings make LMS-1 an important contributor to the stellar population of the inner Milky Way halo.
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